On Sun, 2008-05-18 at 19:27, Joe Subich, W4TV wrote: ... > Using a excessively narrow filter on RTTY is a balancing act. > > If the bandwidth is too narrow the tones tend to become "smeared" > - the transitions become less easy to detect (in other words, > the "eye" begins to close). However, narrowing the bandwidth > also reduces the amount of QRM/QRN making it through the filter > and to the decoder/detector.
How about using a Nyquist filter? A Nyquist filter is one that results in no inter-symbol interference. That is, the filtered output is guaranteed to pass exactly through the nominal symbol location at the middle of each symbol, no matter what data may have been sent in adjacent symbols. So you get the narrowest possible bandwidth without "smearing" the signal, "closing the eye" or whatever you want to call it. For that to work, the total response must be Nyquist, including both the filter in the transmitter as well as in the receiver. Many commercial systems use "Root Nyquist" filters in both the transmitter and receiver. Since each filter's frequency response is the square root of a Nyquist filter response, the total system response is Nyquist, resulting in no inter-symbol interference. However, most RTTY transmitters do not have such a filter. Typically they just use a single-pole R-C low-pass filter to limit the transmitted bandwidth. However, if the time constant of the R-C filter were known, it would be straightforward to design a receive filter that results in a net Nyquist response. The problem is that different RTTY transmitters use different amounts of filtering. However I suspect that a "Nyquist" receive filter designed to compensate a typical/average transmit filter probably would have pretty good performance on most received signals. Does anyone know if this has ever been tried? Al N1AL _______________________________________________ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
